Device for damping vibration of long steam-turbine blades



Dec. 6, 1966 EllCHl ISHIBASHI DEVICE FOR DAMPING VIBRATION OF LONG STEAM-TURBINE BLADES Filed April 7, 1966 ATTORNEY United States Patent 3,290,004 DEVICE FOR DAMPING VIBRATION 0F LONG STEAM-TURBINE BLADES Eiichi Ishibashi, Hitachi-shi, Japan, assignor to Hitachi, Ltd., Tokyo, Japan, a corporation of Japan Filed Apr. 7, 1966, Ser. No. 540,973 Claims priority, application Japan, Apr. 9, 1965, 40/ 20,544 1 Claim. (Cl. 253-77) This invention relates to the vibration damping of steam-turbine blades and more particularly of long shrouded type blades and has for its object to minimize the stress caused in the material of long blades by vibration to which they are subjected thereby to prevent their otherwise occurring failure.

In recent years, with increase in the output of steam turbines, the length of blades in low-pressure stages has been increased markedly, making it necessary to impart to such blades a sufficient strength to withstand huge centrifugal forces While allowing them to vibrate only in modes outside the normal range of turbine speed.

The present invention is intended to cope with the above situation and proposes to design long turbine blades and particularly the extreme end portions thereof relative to the shrouding structure so as to obtain a large mechanical damping effect thereby to minimize the vibrational stress caused in the material of the blades.

According to the present invention, such damping characteristic is obtained by utilizing the drain formed on the turbine blades and previously regarded as entirely useless and harmful by introducing it as a damping fluid into a space provided between the blade end and the adjacent shroud portion.

The foregoing and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a fragmentary sketch of the tip portion of a typical conventional steam-turbine blade;

FIG. 2 is a cross-sectional view taken along the line AA in FIG. 1; and

FIGS. 3 to 7 illustrate one preferred embodiment of the present invention; FIG. 3 is a fragmentary sketch of the blade tip; FIG. 4 is a fragmentary transverse crosssectional view of the shrouded blade assembly; FIG. 5 is a fragmentary perspective view of the blade tip, showing recessed grooves for drain separation and small drain apertures formed in the blade; FIG. 6 is a cross-sectional view taken along the line B-B' in FIG. 5 and FIG. 7 is an explanatory cross-sectional view of the blade assembly lustrating the flow of drain therein.

In cases where long turbine blades are shrouded, the shroud 3 is usually aflixed to the blade 1 by upsetting tenons 2 formed thereon, as shown in FIG. 2. Such tenon structure, however, involves a disadvantage that heavy stresses inevitably evolve in the tenons 2 under centrifugal forces since they cannot be made with any satisfactorily large cross-sectional area. If the tenons were designed large enough, the adjacent portions of the shroud would be undesirably reduced in effective crosssectional area.

According to the present invention, the shroud ring is partly formed integral with the blade 8, as indicated at 4 in FIG. 3, by machining. The shroud section 4 can obviously be designed to have a large strength compared with the conventional tenon structure. Formed in the shroud section 4 is a number of small apertures, some of 3,290,004 Patented Dec. 6, 1966 which are shown in FIG. 3 at 5, to serve as passageways for directing the drain formed on the blade and collected through grooves 6 formed therein into gaps between the flanged blade end or shroud section 4 and the remainder of the shroud structure.

FIG. 4 fragmentarily illustrates a typical shrouded blade system employing shroud sections 4 formed integral with respective blades 8, in cross section at right angles to the turbine shaft. The main shroud ring structure in this case is composed of shroud sections 9 and an annular plate 10, which are fixedly joined together by rivets 11, as shown, or alternately by screw or spot-welding means. In order to attain the object of the present invention, the shroud structure is so designed that, when assembled together with the flanged blades 8, a fine circumferentially extending space or gap 12 is formed between the end face of each of the blades or the shroud section 4 thereon and the adjacent inner surface of the annular plate 10 and fine radially extending spaces or gaps 13 and 14 are formed between the flanged tip of each blade 8 or the shroud section 4 integral therewith and the two adjacent shroud sections 9 joined to the annular plate 10.

Referring next to FIGS. 5 and 6, grooves 6 formed in the blade adjacent to its extreme end and in the vicinity of the steam inlet side of the blade extend substantially radially or along the axis of the blade to serve the purpose of drain separation, and the drain formed in the flow of steam is mostly collected in such grooves, as indicated by the dotted arrows 7 in FIG. 6.

Though, in the past, steam drain collected on turbine blades has been regarded as useless and harmful and removed exteriorly of the turbine casing, it is utilized according to the present invention as a vibration-damping fluid to serve between the flanged blade 8 and the main shroud structure 9-10. For this purpose, small apertures 5 are formed in the end face of the blade 8 or in the shroud section 4 integral therewith to communicate with the drain grooves 6 formed in the side of the blade. Re ferring to FIG. 7, the steam drain collected in the grooves 6, as indicated by the dotted arrows 7, flows radially out wardly along the grooves, as indicated by the arrow a, and through the apertures 5 into the gap 12 between the shroud section 4 and the annular plate 10 and further into gaps 13 and 14, as indicated by the arrows b and c to act upon the blade 8 as a powerful damping fluid. Another important advantage of this arrangement is that the frictional drag acting between the shroud sections 4 and 9 also has a substantial damping effect, as will be readily understood.

It will be appreciated from the foregoing that the device of the present invention has a high vibration-damping effect and is highly valuable in the industry.

What is claimed is:

A device for damping vibration of shrouded long steam-turbine blades comprising a spacing provided between the extreme end portion of the blade and the associated shroud structure and means for directing steam drain into said spacing.

References Cited by the Examiner UNITED STATES PATENTS 1,829,674 10/ 1931 Rosenlocher 25 376 2,221,678 ll/ 1940 Hechman 253-77 2,292,072 8/ 1942 Hanna et a1 25 3-77 2,349,187 5/1944 Meyer 253--77 MARTIN P. SCHWADRON, Primary Examiner.

E. A. POWELL, ]R., Assistant Examiner. 

